1. Field of the Invention
The present invention relates to an ultra precise polishing method and an ultra precise polishing apparatus used for polishing a workpiece surface.
2. Description of the Related Art
In optical glass components such as a lens, there are a spherical lens and an aspheric lens, which are rotationally axisymmetric, and a free formed lens, which is rotationally nonaxisymmetric, etc., and press molding using a molding die serves as a leading processing means for mass-production of these lenses. In this case, if the forms of the lenses press-molded by a molding die are not precise, a function as an optical instrument deteriorates for aberration is produced in optical instruments such as a camera in which the lens is incorporated. When the surface roughness of the molding die is rough, the surface roughness of the molded lens also becomes rough surface, generating a flare in the optical instrument. Thus, although processing precision of a lens is important in press molding of the lens, what influences the processing precision of the lens is form precision of the surface of the molding die imprinted on the lens.
Laid-open Japanese Patent Application No. H8-120470 describes a method for ultra precise polishing of a surface of a workpiece such as a molding die by irradiation with a gas cluster ion beam. When the gas cluster of the ion beam is split by the collision with the workpiece caused by irradiation of the workpiece, multi-body collision occurs between cluster component atoms or molecules and workpiece component atoms or molecules, and motion in a horizontal direction with respect to the workpiece surface is intensified. As a result, the convexity of the workpiece surface is largely smoothened, and ultra precise polishing producing evenness on an atomic scale can be achieved.